Tongue-eating louse (Cymothoa exigua)
The tongue-eating louse, is a parasitic crustacean of the family Cymothoidae. This parasite enters fish through the gills, and then attaches itself at the base of the fish’s tongue. The female attaches to the tongue and the male attaches on the gill arches beneath and behind the female. Once in the mouth, it extracts blood through the claws on its front, causing the tongue to atrophy from lack of blood. The parasite then replaces the fish’s tongue by attaching its own body to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue. It appears that the parasite does not cause any other damage to the host fish.Once C. exigua replaces the tongue, some feed on the host’s blood and many others feed on fish mucus. This is the only known case of a parasite functionally replacing a host organ.